1. Field of the Invention
The present invention relates to an exposure apparatus for exposing a substrate to radiation such as light, and a device manufacturing method using the exposure apparatus.
2. Description of the Related Art
A conventional projection exposure apparatus transfers and projects a circuit pattern of a reticle (mask) onto a wafer through a projection optical system in manufacturing a fine semiconductor device, such as a semiconductor memory and a logic circuit, using the photolithography technology.
In order to improve the resolution and expand the exposure area, a recent mainstream projection exposure apparatus is a step-and-scan exposure apparatus (i.e., a scanner) that scans a reticle relative to a wafer, and exposes a reticle pattern onto each exposure area on the wafer.
The scanner that has one wafer stage measures a wafer's surface shape by a focus measurement sensor FS arranged ahead in the scan direction in exposing a predetermined area PA with slight-shaped exposure light EL, as shown in FIG. 12. The simultaneous measurement of the wafer's surface shape and exposure provides real-time and precise focus control. This focus control arranges the focus measurement sensor FS ahead of the exposure position (projection optical system) and vertically adjusts the wafer stage based on a measurement result of the focus measurement sensor FS, thereby focusing the wafer at an exposure position. The measurement position of the focus measurement sensor FS and the exposure position are fixed. Here, FIG. 12 is a schematic plane view showing a relationship between a focus measurement position and an exposure position on a wafer.
On the other hand, an exposure apparatus having plural wafer stages is proposed for improved productivity. See Japanese Patent Application No. 2000-323404. This exposure apparatus previously measures the wafer's surface shape, and reflects this measurement result on the exposure. In general, for improved processing efficiency, a driving amount in the Z direction during exposure is calculated or mapped based on the wafer's surface shape previously measured by the wafer stage, and the wafer is exposed on the other stage based on the driving amount.
However, the conventional exposure apparatus cannot precisely measure the wafer's surface shape, or provide precise focus control. This is because there is an area that causes an error in a measurement result (measurement error area) due to a (primary coat) circuit pattern formed on a wafer, which is a pattern previously formed on a substrate and under a photoresist coated thereon.
FIGS. 13A and 13B are illustrations in which a focus measurement result has an error due to the primary coat circuit pattern. For example, a focus measurement system that uses an obliquely incident optical system introduces the measurement light obliquely to the wafer's surface, and calculates a focus value using a position of the reflected light. This focus measurement system detects the reflected light intensity, and calculates a position of the reflected light by calculating a centroid position of a detected waveform. When the measurement light ML is irradiated onto a boundary between a high reflectance part HR and a low reflectance part LR, as shown in FIG. 13A, the reflected light intensity from the high reflectance part HR is higher than that from the low reflectance part LR. Thereby, the detected waveform deforms, as shown in FIG. 13B, from an ideal waveform IW to a waveform RW. As a result, a measurement result shifts to HD in a direction of the high reflectance part HR from an original centroid position GP, causing an error in a focus measurement value.
Such a measurement error area exists, for example, as a wiring pattern CP in a shot shown in FIG. 14. When a surface shape is determined from a precisely measured measurement point MP and a measurement point MP′ that is measured with a measurement error, defocusing and chip failure occur. Here, FIG. 14 is a view for explaining a measurement error area in a shot.
Accordingly, the conventional exposure apparatus does not measure the measurement error area or use a measurement result in the measurement error area. More specifically, there are proposed a method that arranges many focus measurement sensors and selects a focus measurement sensor to be used, a method that shifts a measurement position through a shift of a focus measurement sensor or an optical system, and a method that removes an abnormal value from a measurement result.
The method that arranges many focus measurement sensors is problematic due to the increased cost and lack of arrangement space. The method that shifts the focus measurement sensor or optical system complicates an optical system. Moreover, the method that considers a measurement value exceeding a threshold to be abnormal, and removes or invalidates this measurement value has no measurement values in the measurement error value in the shot, if such a measurement error value to be invalid exists. As a result, the number of measurement points in the shot decreases, and the calculation precision of the wafer's surface shape deteriorates.